Although pH is one of the most fundamental properties of the tears, little is known about its diurnal variations. Indeed, the wide range of values appearing in the literature of human tears has resulted primarily from one point in time determinations. Among the specific goals of this study are: 1) to systematically monitor and to expand pilot findings on an individual eye basis of the diurnal course of tear fluid pH. Such individual profiles will then be classified by such quantitative criteria as cycloid period, slope, amplitude and phase, as well as by diurnal mean, standard deviation from the mean, best fit slope for the day and probability of finding the tear pH value within particular range limits of the diurnal mean or best fit diurnal slope: and 2) to investigate the nature of tear pH stability and change in relation to tear buffer capacity, periods of prolonged eye closure, blink rate, environmental characteristics and foreign body response, monitoring the change course in each case on continuing short interval bases. To minimize contact with the atmosphere, tear samples will be passively drawn by microcapillary from within the conjunctival cul de sacs, immediately sealed until evacuated against a KCl fluid bridge into a closed chamber, temperature stabilized microelectrode system for measurement. The long-term objective of this study, then, will be to describe and where possible to categorize, through continuing short-term interval analyses, the diurnal nature of tear pH change as it occurs on an individual eye basis. From such data will then be developed a population description as well as individual models of human tear pH, which could serve as a physiological baseline against which pathological disturbances might be better gauged, ophthalmic pharmaceuticals more effectively formulated and pH sensitive therapeutic bandage lenses better designed.